SQL : select data with null value on end date if other record has the same start date

Question

I am currently working with SQL server 2008. I have multiple records that have students enrolled in the same course but some records have the same start date and null value or an actual date for the end date. I am trying to get the records with the null value if the start date is the same for the same student. The data in the system also contains records with a start and end date but I am intending to keep those records. I have tried the query below but it is not giving me back any results. Is there a way to accomplish this? Any help would be appreciated.

SELECT DISTINCT
  t1.*
FROM Enrolled_students t1
JOIN Enrolled_students t2
  ON t1.studentid = t2.studentid
  AND t1.program_enrolled = t2.program_enrolled
  AND t1.startdate = t2.stardate
  AND t1.enddate <> t2.enddate

DATA

StudentID    program    StartDate            enddate_Date
267342      Science   2016-09-19 00:00:00.000    NULL
267342      science   2016-09-19 00:00:00.000   2017-01-17 00:00:00.000 
435359      math      2017-05-18 00:00:00.000   2017-08-29 00:00:00.000
290332      Lab       2014-02-11 00:00:00.000   NULL 

RESULTS

StudentID    program    startDate            end_Date
267342      Science   2016-09-19 00:00:00.000    NULL
435359      math      2017-05-18 00:00:00.000   2017-08-29 00:00:00.000
290332      Lab       2014-02-11 00:00:00.000   NULL 

Answer 1

Run the below SQL. this will return you the Student records with End Date which has another entry in the same table for the same student+course+start date combination with NULL in the end date

SELECT
*
FROM Enrolled_students 
    WHERE enddate_Date IS NOT NULL
        AND EXISTS
        (
            SELECT
                1
                FROM Enrolled_students  ES
                    WHERE ES.StudentID = Enrolled_students.StudentID
                        AND ES.Program = Enrolled_students.Program
                        AND ES.StartDate = Enrolled_students.StartDate
                        AND ES.enddate_Date IS NULL
        )

Answer 2

You can try this.

SELECT * FROM Enrolled_students E1
WHERE NOT EXISTS
    ( SELECT * FROM Enrolled_students E2 
        WHERE E1.StudentID = E2.StudentID
            AND E1.enddate_Date IS NOT NULL 
            AND E2.enddate_Date IS NULL )

Result:

StudentID   program              StartDate               enddate_Date
----------- -------------------- ----------------------- -----------------------
267342      Science              2016-09-19 00:00:00.000 NULL
435359      math                 2017-05-18 00:00:00.000 2017-08-29 00:00:00.000
290332      Lab                  2014-02-11 00:00:00.000 NULL